The present invention generally relates to synchronous generators, and more specifically, to an apparatus and method for detecting a brush liftoff in a synchronous generator rotor circuit.
Synchronous electrical generators (“synchronous generators”) are used in electric utility systems to convert mechanical rotation (e.g., shaft rotation provided by a steam turbine) into alternating electric current via well-known electromagnetic principles. After suitable conditioning, the alternating electrical current is typically transmitted and distributed to a variety of power system loads.
In general, synchronous generator design is based on Faraday's law of electromagnetic induction and includes a rotating rotor driven by an external torque for inducing an electromagnetic field (EMF) in a stationary stator. The rotor includes a field winding wrapped around a rotor body, and the stator includes an armature winding wrapped around an armature body.
In operation, a direct current is made to flow in the field winding to generate a magnetic field. When the rotor is made to rotate, the magnetic field rotates with it, passing through the stator winding(s) and inducing an electric current therein.
Insulation material is utilized to cover the field winding(s) in order to electrically isolate the field winding(s) from the rotor body. As is known, detecting a loss of the insulation covering the field winding(s), or a field ground, is vital to ensuring reliable operation of the synchronous generator. While a field ground of insulation at one point along the field winding may be inconsequential to operation, field grounds at two locations along the field winding could result in serious damage to the synchronous generator.
One method used to achieve a field-ground protection is the so-called switched-DC injection method. One implementation of a switched-DC injection method is illustrated on page 61 of a Siemens AG instruction manual, entitled “Numerical Machine Protection 7UM515,” version v3.1, dated 1996, the complete disclosure thereof being incorporated herein by reference. A variation of the switched-DC injection method is also described in U.S. Pat. No. 6,794,879, entitled “Apparatus and Method for Detecting and Calculating Ground Fault Resistance,” issued on Sep. 24, 2004, naming Lawson et al. as inventors, the complete disclosure thereof being incorporated herein by reference. Unlike other switched-DC injection methods where a voltage is measured across a grounded “sense resistor” in order to determine a loss of field winding insulation, the '879 patent discloses using a voltage controlled oscillator (VCO) configured to measure a differential voltage across a floating sense resistor.
The switched-DC injection method requires that the rotor body be connected to electrical ground. This is typically accomplished by means of a (rotor) grounding brush. When the grounding brush no longer provides a low resistance circuit between the rotor body and electrical ground, “brush liftoff” has occurred, and the device performing the field-ground protection is no longer able to perform its task. Detecting such a brush liftoff, or open condition of the grounding path provided by the grounding brush, is therefore critical to reliable synchronous generator operation. The present invention provides such protection.